Cell surface receptors for ATP can be divided into metabotropic (P2Y/P2U) and ionotropic (P2X) classes. The metabotropic class belongs to the superfamily of G protein-coupled receptors, with seven transmembrane segments. The ionotropic class members (P2X1-P2X6) are ligand-gated ion channels, currently thought to be multisubunit proteins with two transmembrane domains per subunit (Buell et al, Europ. J. Neurosci. 8:2221 (1996)). P2Z receptors have been distinguished from other P2 receptors in three primary ways (Buisman et al, Proc. Natl. Acad. Sci. USA 85:7988 (1988); Cockcroft et al, Nature 279:541 (1979); Steinberg et al, J. Biol. Chem. 262:3118 (1987)). First, activation of P2Z receptors leads not only to an inward ionic current, but also to cell permeabilization. Second, 3′-O-(4-benzoyl)benzoyl ATP (BZATP) is the most effective agonist, and ATP itself is of rather low potency. Third, responses are strongly inhibited by extracellular magnesium ions, that has been interpreted to indicate that ATP4- is the active agonist (DiVirgilio, Immunol. Today 16:524 (1995)).
A seventh member of the P2X receptor family has been isolated from a rat cDNA library and, when expressed in human embryonic kidney (HEK293) cells, exhibits the above three properties (Surprenant et al, Science 272:735 (1996)). This receptor (rP2X7) thus corresponds to the P2Z receptor. rP2X7 is structurally related to other members of the P2X family but it has a longer cytoplasmic C-terminus domain (there is 35-40% amino acid identity in the corresponding region of homology, but the C-terminus is 239 amino acids long in the rP2X7 receptor compared with 27-20 amino acids in the others). The rP2X7 receptor functions both as a channel permeable to small cations and as a cytolytic pore. Brief applications of ATP (1-2s) transiently open the channel, as is the case of other P2X receptors. Repeated or prolonged applications of agonist cause cell permeabilization reducing the extracellular magnesium concentration potentiates this effect. The unique C-terminal domain of rP2X7 is required for cell permeabilization and the lytic actions of ATP (Suprenant et al, Science 272:735 (1996)).
The P2Z/rP2X7 receptor has been implicated in lysis of antigen-presenting cells by cytotoxic T lymphocytes, in the mitogenic stimulation of human T lymphocytes, as well as in the formation of multinucleated giant cells (Blanchard et al, Blood 85:3173 (1995); Falzoni et al, J. Clin. Invest. 95:1207 (1995); Baricolrdi et al, Blood 87:682 (1996)). Certain functional differences exist between rodent and man (Hickman et al, Blood 84:2452 (1994)). The human macrophage P2X7 receptor (P2X7) has now been cloned and its functional properties determined (Rassendren et al, J. Biol. Chem. 272:5482 (1997). When compared with the rat P2X7 receptor, elicited cation-selective currents in the human P2X7 receptor required higher concentrations of agonists, were more potentiated by removal of extracellular magnesium ions, and revised more rapidly on agonist removal. Expression of chimeric molecules indicated that some of the differences between rat and human P2X7 receptors could be revised by exchanging the respective C-terminal domains of the receptor proteins.
It has been reported that certain compounds act as P2X7 antagonists. For example, WO99/29660 and WO99/29661 disclose that certain adamantane derivatives exhibit P2X7 antagonistic activity having therapeutic efficacy in the treatment of rheumatoid arthritis and psoriasis. Similarly, WO99/29686 discloses that certain heterocyclic derivatives are P2X7 receptor antagonists and are useful as immunosuppressive agents and treating rheumatoid arthritis, asthma, septic shock and atheroscelerosis. Finally, WO00/71529 discloses certain substituted phenyl compounds exhibiting immunosuppressing activity. All of the references described herein are incorporated herein by reference in their entirety.
A need therefore exists for therapeutic agents, and corresponding pharmaceutical compositions and related methods of treatment, that address the conditions causally related to aberrant P2X7 activity, and it is toward the fulfillment and satisfaction of that need, that the present invention is directed.